This invention relates to a dual mode drum brake device which functions as a leading trailing type (LT type) when a service brake is applied and functions as a duo servo type (DS type) when a parking brake is applied. This type of drum brake device has been disclosed for example by the U.S. Pat. No. 5,070,968, the descriptions of which are incorporated herein by reference.
A conventional dual mode drum brake device is explained with reference to FIGS. 8 and 9. Such a conventional device comprises a pair of brake shoes b, c which are mounted on a back plate a; a hydraulic cylinder e which is arranged between the upper side of the brake shoes b, c in order to pressurize the brake shoes b, c to spread them open with an anchor block d as the fulcrum; a pivot lever f which is pivotally superimposed under the left brake shoe b; a shoe clearance adjustment device g adjacent to the hydraulic cylinder e which is extended between both brake shoes b, c and functionally engages with the upper portion of the pivot lever f and the right brake shoe c; a strut h adjacent to the anchor block d, which is extended between both brake shoes b, c; and a brake lever i which engages with the lower portion of the pivot lever f and the left side of the strut h.
The strut h is spread into two forked legs toward its left end. The brake lever i is pivotally mounted on one of forked legs at a side adjacent the back plate a. A parking brake is designed to be activated by pulling a parking brake cable j connected with the end of the brake lever i.
However, the above-described conventional dual mode drum brake device has the following points which need to be improved.
A through hole for the parking brake cable j needs to be formed near the anchor block d. However, this portion of the back plate needs to have the highest strength among other portions of the back plate a. Yet, securing the through hole possibly reduces the strength of the back plate a. Due to the decreased strength of the back plate a, there is a need for a supplemental means to increase the strength of the back plate a, e.g., by adding a reinforcing plate or designing the back plate a to be thicker. However, adding the supplemental means and designing the back plate a to be thicker raise the weight and cost of the device.
The parking brake cable j is arranged at the lower portion of the back plate a. Hence, as the position where the parking brake cable j is arranged comes closer to the road surface, it becomes more likely to be damaged by being hit by flying gravel or by colliding with obstacles on the road surface.
The entire brake lever i is to be located inside the brake device, and a cable end k of the parking brake cable j must be pre-connected with the brake lever i. These requirements make the connecting work more difficult and the handling of the brake device more inconvenient.
This invention was made to improve the above points and to that end this invention provides a dual mode drum brake device which does not have a reduced strength of the back plate near the anchor block and which does not have the need for a supplemental means to increase the strength of the back plate.
Another aspect of this invention is to provide a dual mode drum brake device which enables a design of the arrangement of the parking brake cable away from the road surface, thereby minimizing the possibility of damage to the parking brake cable.
This invention is a dual mode drum brake device, in which a pair of brake shoes are provided on a back plate to face each other; a service brake actuator which is adapted to be activated upon an activation of a service brake is arranged between adjacent upper facing ends of the brake shoes; an anchor block is arranged between adjacent lower facing ends of the brake shoes; a pivot lever is arranged such that a central region of said pivot lever is pivotally mounted at a central region of one of the brake shoes; a shoe clearance adjustment device is arranged adjacent to the service brake actuator and extends between the pair of brake shoes and functionally engages the upper portion of the pivot lever; a strut is extended between the lower portion of the pivot lever and the other one of the brake shoes and functionally engages with the lower portion of the pivot lever; and the brake shoes are designed to be able to spread open upon an activation of a parking brake, wherein: a parking brake actuator which is adapted to activate upon the activation of the parking brake is arranged adjacent to the service brake actuator; and an operating portion of the parking brake actuator extends outside the brake through the back plate.
This invention further is a dual mode drum brake device as above, wherein the operating portion of the parking brake actuator which activates upon the activation of the parking brake is arranged to be movable in an almost right angle relative to the back plate.
This invention still further is a dual mode drum brake device as above, wherein the operating portion of the parking brake actuator which activates upon the activation of the parking brake is arranged to be movable almost parallel to the back plate.
This invention still further is a dual mode drum brake device as above, wherein a cushioning member is arranged between the anchor block and the brake shoes.
This invention still further is a dual mode drum brake device as above, wherein a protuberance is integrally formed by pressing on the central region of the pivot lever or on the central region of the one of the brake shoes; and the protuberance is pivotally supported in a hole which is provided on the other of the pivot lever or the one of the brake shoes.
This invention still further is a dual mode drum brake device as above, wherein when a central pivot section of the pivot lever is a pivot point, a resistance force against the opening of the brake shoes during a parking brake operation is designed to be greater on a side of the brake shoes adjacent the anchor block than on a side of the brake shoes adjacent the service brake actuator.
This invention still further is a dual mode drum brake device as above, wherein the shoe clearance adjustment device extended between the brake shoes is adapted to automatically extend when sensing an excessive amount of opening of the pair of brake shoes during a service brake operation and to adjust clearance between a brake drum and linings of the brake shoes.
This invention yet further is a dual mode drum brake device as above, wherein the strut extended between the lower portion of the pivot lever and the lower portion of the other one of the brake shoes is adapted to automatically extend when sensing an excessive amount of opening of the pair of brake shoes and the pivot lever during service brake operation and to adjust an application stroke of the parking brake actuator.